An oil drop of the radius $2 \mathrm{mm}$ with a density $3 \mathrm{g}$ ${\mathrm{cm}}^{-3}$ is held stationary under a constant electric field $3.55\times {10}^5 \mathrm{V} {\mathrm{m}}^{-1}$ in the Millikan's oil drop experiment. What is the number of excess electrons that the oil drop will possess? $\left(\text{consider} g=9.81 \mathrm{m} {\mathrm{s}}^{-2}\right)$.

Shown in the figure is a shell made of a conductor. It has inner radius $a$ and outer radius $b$, and carries charge $Q$ . At its centre a dipole $\overrightarrow{\mathit{p}}$ is placed as shown then:

Three charges are placed at the three vertices of an equilateral triangle of side $a$ as shown in the figure. The force experienced by the charge placed at the vertex $A$ in a direction normal to $BC$ is